Bio-sensing is usually performed by measuring a parameter such as the refractive index of a liquid which is dependent on the concentration of a solute measurand. Conventional methods require an absolute measurement of the refractive index detected through a change in transmission properties, for example the Surface Plasmon Resonance angle. Such measurement can be quite difficult to implement and requires a lot of equipments generally complicated to use. An optical fiber-based interferometer may be used but the core region of the fiber(s) is difficult to access since it is completely enclosed; such an interferometer is very unstable due to the nature of the fiber.
A Surface Plasmon-Polariton (SPP) represents a surface electromagnetic wave that propagates between two media having respective permittivity real parts of opposite signs and made, for example, of respective dielectric and metallic materials [1]. SPP can be supported on cylindrical and planar surfaces or geometries. The SPP field components have their maxima at the interface between the media and the metal layer and decay exponentially in both media [1]. Their small penetration depth in the media makes SPPs a great tool for sensor applications.
Traditional planar SPP sensor systems used for bio-sensing work on the principle of prism coupling by altering the angle of the incident beam to match the propagation constant of the SPP. This relies on incorporating moving parts into the sensor.
Other known optical fiber sensors are based on the properties of the SPP penetrating along the surface of a thin gold or silver layer deposited on an exposed portion of the core of the optical fiber [2-10]. In the latter optical fiber sensors, the core of the fiber is used instead of the coupling prism of the traditional planar SPP sensor systems. More specifically, these fiber sensors are constructed by modifying traditional SPP planar sensor systems. Scanning the light wavelength or change in the angle of incidence of the incident light is used for SPP excitation. Generally, the resulting optical fiber sensors include moving parts which render the SPP excitation process rather difficult.
Therefore, there is a need for an improved optical fiber sensor using SPP, which is simple, free from instability and demonstrates a high efficiency whereby it can be used as a successful bio-sensor.